The present invention relates generally to numerical control lathes, and more particularly, to an apparatus and method for lathe tool calibration, i.e., for determining relative position of the cutting edge on a numerical control lathe tool used for turning, facing, boring, or similar operations.
Lathes are well known and have been used in machining for many years. Lathes necessarily include a base or frame, a spindle chuck or other holder for holding and rotating a workpiece about an axis, and a turret or other holder for securing one or more cutting tools on the lathe frame. The cutting tool itself, in its tool holder, is generally movable in two directions transverse to and parallel to the axis of rotation of the spindle chuck in order to accomplish the machining operation on the workpiece.
Automated machine tools such as turret lathes are also commonplace, employing a control system commonly referred to as a numerical control or a computer numerical control. Explained simply, these controls function to command movement of the cutting tool or the workpiece itself per previously programmed numerical instructions to produce a finished product that is machined to precise dimension and configuration. Control of workpiece and tool holder speeds, feeds and various other functions are handled by the numerical control through proper programming.
A major problem of numerically controlled lathes is their inability to monitor location of the point or edge on the cutting tool. Although the numerical control does constantly monitor the movement of the cross slide and carriage of the lathe, it cannot monitor the actual position of the tool point. With conventional NC lathes, as they are often called, the programmer has to know the positions of these tool points relative to a certain fixed point on the turret, or other tool holder, for all of the cutting tools being used in the machining operation. The programmer uses this information in programming the movement of cross slides and carriages on the lathe to accomplish the desired machining.
One conventional method to achieve tool setting and alignment has been to use only so-called "qualified tools." This term refers to a combination of dimensionally controlled tool bits and tool holders which are available on the market, having their dimensions predetermined by the manufacturer. The accuracy of this type of tool is such that for ordinary machining, it is probably sufficient to use the nominal dimensions of the tools which are published by the tool manufacturer in programming the numerical control. The problem with this method is that tool selection is limited, and that the attainable accuracy in the machining is dependent upon the degree of accuracy observed by the manufacturer. In addition, there is no way for the manufacturer to account for deviations caused by tool wear, mounting misalignment by the programmer and the like.
A second conventional method has been to calibrate each cutting tool at what is normally referred to as a tool presetting gage. This is done on an off-line basis at a location away from the lathe. Examples include the disclosures in U.S. Pat. No. 3,490,318 issued to McKenzie Jones, U.S. Pat. No. 3,888,015 issued to Williams, U.S. Pat. No. 3,578,868 issued to Wopkemeier, and U.S. Pat. No. 3,580,129 issued to Austin. Besides being time consuming, these methods have the inherent problem of inaccuracies resulting from the cutting tool being measured at some remote gauging stand and then being moved to the lathe where it is again mounted in a turret or other holding device.
A third conventional method has been to make a small cut on the workpiece with the cutting tool, and then to measure this diameter to establish the relative radial position of the tool point against the tool post or holder. In a horizontal bed lathe, this measurement corresponds to the X-axis calibration coordinate. For calibration in the lengthwise direction, i.e., the Z-axis coordinate parallel to the rotational axis of the workpiece, a small cut in the end of the workpiece is taken. These measurements are input to the numerical control which, in turn, automatically compensates for the true position of the cutting edge of the tool. Although calibration is done with the cutting tool on the lathe, this method also suffers from being cumbersome and time consuming because the operator has to make cuts, measure the diameter or lengthwise position of the machined surfaces, and then input this data to the numerical control through keyboard or other means in order to compensate for dimensional errors in the cutting tool. These problems are, of course, magnified when multiple cutting tools are used on the lathe, as is often the case.
A fourth conventional method has been to employ a microscope mounted on a certain fixed point of the lathe. After initially mounting the cutting tool in its holder, the operator locates the tip or point of the tool in the field of view and inputs its relative position into the control system. A problem with this method is once again the complexity of the procedure in addition to the cumbersome mounting and handling of the microscope in most instances.
Tool point deviation detectors are also known to the machining industry. These devices measure or presume tool length, with this length being programmed into the machine control. The control attempts to automatically bring the cutting tool to a predetermined position, whereupon tool length deviation is somehow measured and input back to the control. Detection methods of this kind are disclosed in a patent issued to Rhoades, U.S. Pat. No. 3,704,641 (utilizing an electromagnetic transducer); a patent issued to Kinney, U.S. Pat. No. 3,641,849 (relying upon contact between the cutting tool and a conductive calibration block); and a patent issued to Blazenin et al., U.S. Pat. No. 4,018,113 (which uses a solid state line scan camera). Besides complexity, one disadvantage of these deviation detectors is that tool length must be initially input to the control so that deviation can be detected. This requires prior measurement either before or after the tool is mounted in its holder, and in this way again complicates the job of the operator.